What does doing something for blood mean

Is anyone familiar with the saying "doing something for blood? "I heard it used like this, "I don't teach anymore for fun, I teach for blood" And they seem to mean it in a possitive way.

Thanks in advance.

In your example, it would mean something like 'with a vengeance', to use a different idiomatic phrase. It means the teacher has become annoyed with the students not learning, so he/she is going to get serious with them.

"No more Mr. Nice Guy; this time I'm out for blood."